Material reduction machines are used to reduce larger pieces of material into smaller pieces by cutting, chopping, shredding or breaking. Generally, a material-reduction machine will have an enclosure for a reducing mechanism, such as a rotating disc or drum equipped with blades, knives or hammers. The enclosure will typically have a feed inlet through which the larger materials to be reduced are introduced, and a discharge outlet through which the smaller materials are discharged after reduction. One type of material reduction machine is a wood chipper that is used to reduce trees and, their limbs and branches to wood chips. The use of wood chippers avoids the environmental and other problems associated with burning trees and brush or with depositing them in a landfill. Furthermore, by reducing wood to chips of a useful size, a wood chipper may be employed to produce a valuable chip product. Wood chips can be used as mulch or fuel. They can also be used as raw material for creating a pelletized fuel product or as raw material in a chemical pulp process. Wood chips that are intended for use as fuel or in a pelletizing process may first need to be dried. It is desirable that such chips have a uniform chip thickness and a high surface area to volume ratio. It is also desirable that chips which are intended for use in a pulp process be of a uniform size. Ideal pulp chips fall into a narrow thickness range so that they can cook and delignify uniformly. Long and narrow chips and very small chips are undesirable because they can plug the pulp process screens and overcook, thereby damaging the wood fibers and reducing the strength of the pulp.
Most wood chippers are either disc chippers or drum chippers. Disc chippers include knives mounted on a rotating disc that cut across the grain of the wood stem generally perpendicular to the direction of the grain. Disc chippers create chips of a generally uniform size. However, such chippers do not have the production capacity of drum chippers. Drum chippers include knives mounted around the circumferential wall of a cylindrical drum that cut across the wood feed stock in a path that varies with respect to the orientation of the grain of the feed stock to the drum. In the part of the wood feed stock where the knives encounter the wood near the three o'clock or the nine o'clock position of the drum, depending on the side of entry of the feed stock, the knives pass across the wood in a direction that is perpendicular to the direction of the grain. In the part of the feed stock where the knives encounter the wood nearer the six o'clock position of the drum, the knives pass across the wood in a path that is more parallel to the direction of the grain. Because the cutting path angle relative to the direction of the grain varies in this manner, the chips break from the feed stock differently, with the chips cut by the drum nearer its six o'clock position tending to be longer and more irregular in size.
The cutting drum of a drum chipper is rotated in a housing having only a slightly larger diameter than the arc cut by the leading edges of the knives. Because such drum chippers are known to jam with chips and stall, some such chippers are provided with blowers or augers to release the chips from the knives and propel them into a discharge chute. It is also known to provide a drum chipper having a pocket in the drum associated with each knife. U.S. Pat. No. 5,005,620 describes a drum chipper in which the peripheral wall of the drum defines a spaced pocket behind each knife. Each knife in this assembly is generally centered within its pocket so that chips may enter the pocket on the leading edge side of the knife and exit the pocket on the trailing edge side of the knife. As the drum rotates, wood chips cut by each knife enter the pocket on the leading edge side and pass behind the knife. The wood chips in each pocket remain in the pocket until the drum rotates to align the trailing edge side of the knife with the discharge chute, where the chips are expelled into the chute under the influence of centrifugal force. Despite these improvements in drum chipper technology, it is still the case that drum chippers generally produce a significant fraction of chips that are long and irregular in size. Such chip may not be useful as raw material for pelletizing and chemical pulping processes.
It would be desirable if a material reduction machine such as a drum chipper could be provided that would allow for more control of the size and shape of chips produced. It would also be desirable if such a drum chipper could be adapted to produce wood chips that are suitable for various uses.